Uptake and physiological impacts of nanoplastics in trees with divergent water use strategies

Anthropogenic contaminants can place significant stress on vegetation, especially when they are taken up into plants. Plastic pollution, including nanoplastics (NPs), could be detrimental to tree functioning, by causing, for example, oxidative stress or reducing photosynthesis. While a number of studies have explored the capacity of plants to take up NPs, few have simultaneously assessed the functional damage due to particulate matter uptake. To quantify NPs uptake by tree roots and to determine whether this resulted in subsequent physiological damage, we exposed the roots of two tree species with different water use strategies in hydroponic cultures to two concentrations (10 mg L−1 and 30 mg L−1) of model metal-doped polystyrene NPs. This approach allowed us to accurately quantify low concentrations of NPs in tissues using standard approaches for metal analysis. The two contrasting tree species included Norway spruce (Picea abies [L.] Karst), a water conservative tree, and wild service tree (Sorbus torminalis [L.] Crantz), an early successional tree with a rather water spending strategy. At both exposure concentrations and at each of the experimental time points (two and four weeks), NPs were highly associated and/or concentrated inside the tree roots. In both species, maximum concentrations were observed after 2 weeks in the roots of the high concentration (HC) treatment (spruce: 2512 ± 304 μg NPs per g DW (dry weight), wild service tree: 1190 ± 823 μg NPs per g DW). In the aboveground organs (stems and leaves or needles), concentrations were one to two orders of magnitude lower than in the roots. Despite relatively similar NPs concentrations in the tree aboveground organs across treatments, there were different temporal impacts on tree physiology of the given species. Photosynthetic efficiency was reduced faster (after 2 weeks of NPs exposure) and more intensively (by 28% in the HC treatment) in wild service trees compared to Norway spruce (ca. 10% reduction only after 4 weeks). Our study shows that both, evergreen coniferous as well as deciduous broadleaf tree species are negatively affected in their photosynthesis by NPs uptake and transport to aboveground organs. Given the likelihood of trees facing multiple, concurrent stressors from anthropogenic pollution and climate change, including the impact of NPs, it is crucial to consider the cumulative effects on vegetation in future.

There were no significant differences in shoot-to-root ratios between the controls and the two treatments for both species.For wild service tree there was a slight, though not significant, tendency for a higher shoot-root ratio in both LC and HC exposure concentrations after two weeks whereas after the same incubation time such trends were not visible in Norway spruce.However, some tendency for increased shoot-root ratios in HC were visible for Norway spruce after four weeks.Two-and four-weeks incubation may not be sufficient to induce clear changes in growth patterns, even in seedlings with their relatively high growth rates.The slight tendency we observed for increased shoot-root ratios in wild service tree after two and in Norway spruce after four weeks might indicate relatively reduced C allocation to the roots, which is known to occur when plants are exposed to stressors (Joseph et al., 2020).

Figure S1 .
Figure S1.Evolution of the a) hydrodynamic diameter (nm) and b) zeta potential (mV) of the model Palladium doped polystyrene nanoplastics (PS-Pd-NP) in deionized water (DI) at high particle concentration (30 mg/L) and growth media at low (LC) and high (HC) concentrations (10 and 30 mg/L, respectively) over one week.These time points correspond to the time at which solutions were renewed during the exposure experiments.Error bars represent standard deviation.

Figure S2 :
Figure S2:Photos of experiment set-up where 12 trees of each species were grown in a hydroponic system.The root crown of each tree was inserted into the floating mat through a vertical slot.In this way, only the roots were submerged in the liquid solution, whereas the aboveground tissues remained physically isolated from the nutrient solution and potential nanoplastics contamination.

Figure S3 .
Figure S3.Average nanoplastics spiked addition recovery (%) of PS-Pd-NP dispersions (1 μg/L, 2.5 μg/L, and 5 μg/L) on the different tissues of Norway spruce (NS) and wild service tree (WST) (n=3).Results from all concentrations for each tree organ are graphed together, and error bars represent standard deviations.The red line indicates the average recoveries across all tree organs.

Figure S4 :
Figure S4: Interaction plots for the effect of concentration and time on NPs uptake for leaves, stems and roots.

Figure S5 :
Figure S5: Normalized Pigment Chlorophyll Index (NPCI) of wild service tree and Norway spruce after different exposure times to low (LC) and high concentrations (HC) of plastic nanoparticles.Ctrl: untreated controls.Data shown are mean values (N= 3-9)  SD.

Table S5 :
Estimates and p-values of the linear model for the effects of concentration, time, and species on NPs uptake in leaves, stems and roots.Interactions between concentration-time and concentration-species were included.Significance levels: *** <0.001,** <0.01,* <0.05.

Table S6 :
Dry weight (g) of wild service tree and Norway spruce at the end of the experiments.Data shown are mean values ± SD.

Table S7 :
Water content (%) in the three different tissues for each species and each exposure time calculated as 100% -100 (Dry weight / Fresh weight) Data shown are mean values ± SD.

Table S8 :
Dry weight shoot-to-root ratios for wild service tree and Norway spruce at the end of the experiments.Data shown are mean values ± SD.F-ratios (the ratio of the between group variance to the within group variance) and p-values were assessed by one-way ANOVA.